1. Field of the Invention
This invention relates to a device for retaining an optical element or part, and more particularly to an optical part retaining device which can enhance moisture resistance of an adhesive used for fixing the optical part.
2. Prior Art
In general, optical parts such as lens, prism and the like are fixed to a holding frame through an adhesive. As the adhesive, there have hitherto been proposed ultraviolet curing type adhesives capable of well curing in a short time as described, for example, in Japanese Patent laid-open No. 59-228,615. In this case, the ultraviolet curing type adhesive having a semi-fluidity or an easily plastic-deformable flexibility is shaped into pieces having a given size. Then, a plurality of such adhesive pieces 2 are placed in a holding frame 1 along its inner periphery and an optical part (e.g. lens) 3 is inserted into the holding frame 1 as shown in FIG. 4. After the position adjustment, the adhesive piece is cured under the irradiation of ultraviolet ray to fix the optical part 3 to the holding frame 1.
The use of such an ultraviolet curing type adhesive not only facilitates the position adjustment of the optical part but also can perform the quick fixing operation of the optical part held at the exact position in a short time.
However, the ultraviolet curing type adhesive has a drawback that the moisture resistance is poor. For instance, the test for moisture resistance is performed as a link in the chain of the guarantee for the product quality. In such a test, the ultraviolet curing type adhesive for bonding the optical part to the holding frame absorbs a large amount of water to cause swelling, decomposition, interface peeling and the like, resulting in the decrease of adhesion force. Further, the decomposed adhesive flows out or flys in all directions, and consequently it may adhere to the outer surface of the optical part (see symbol A in FIG. 4) to considerably degrade the optical properties. Although the moisture test has hitherto been performed at 49.degree. C. and 95% RH for about 240 hours, severer test conditions of high temperature and humidity are gradually required, which considerably exerts a great influence on the optical properties.